1. Field of the Invention
The present invention, the Moisture and Density Detector (MDD), relates to an apparatus and method for detecting the moisture content and/or density of dielectric materials.
2. Related Art
Moisture Estimation Using Radio Frequency Signals
Several devices have been developed to measure moisture in materials. These devices are based primarily on resistance and capacitance principles. Resistance is the opposition of a body or substance to a current passing through it. Capacitance is the property of a circuit element that permits it to store charge. For resistance devices, a direct current (DC) radio frequency signal is passed through a dielectric material (a material that does not conduct electricity) and the signal strength is measured as a function of the resistance of the material. This resistance measurement is then converted to a moisture content value using correction factors for temperature and species. Capacitance devices measure capacitance value or “power-loss” and estimate moisture content based on known correlation.
U.S. Pat. No. 4,259,633 to Rosenau describes a resistance moisture content estimation technique. The technique applied by Rosenau and others is limited in that it requires that metal pins be inserted into the wood sample being tested. In addition, the electrolytic polarization effects when using DC voltage can result in measurement error. Inserted-pin resistance devices are considered to provide inaccurate estimates when the wood moisture content is above the fiber saturation point of 24 to 30 percent.
U.S. Pat. No. 3,600,676 to Lugwig et al. teaches the capacitance technique whereby an alternating current (AC) radio frequency capacitance device was developed using adjacent electrodes and resonance to determine the moisture content of bulk materials (i.e., coal, chips, etc.). This device applies a range of frequencies to the dielectric material adjacent to the electrodes. The frequency with maximum signal strength is termed the resonant frequency and is a direct function of the moisture content of the dielectric material. The Lugwig et al. device determines the resonant frequency at which signal strength (amplitude) reaches a maximum. Applicant's invention also applies a range of frequencies to the dielectric material and measures the signal strength of each in terms of amplitude. However, Applicant's invention does not determine the resonant frequency but rather relates the measured amplitude of each frequency to predetermined values to determine the moisture content of the dielectric material. In addition, in contrast to Applicant's invention, the Lugwig et al. device does not use phase shift as additional information to estimate moisture content or density.
U.S. Pat. No. 4,616,425 to Burns describes an opposed electrode device based on resistance or capacitance controlled oscillator circuits. Whether based on resistance or capacitance, this device requires conversion to a frequency-dependent DC voltage. Signal strength of the DC voltage is related to predetermined voltage values for the dielectric material to allow moisture content estimation. Direct contact with the dielectric material is required. In contrast to Burns, Applicant's invention does not employ conversion from AC signal to DC signal. In addition, direct physical contact with the wood surface is possible, but not necessary. Furthermore, Burns does not use measurement of phase shift to improve the moisture content estimate. The Burns device also has no capability to estimate dielectric material density.
U.S. Pat. No. 3,430,357 to Perry discloses an opposed electrode device that measures capacitive impedance and associated moisture content in a stack of lumber in a dry kiln. The resistance between a capacitance probe inserted several courses of lumber above a ground electrode gives a measure of stack moisture content in the lumber between the electrodes. This method requires direct contact between the capacitance probe and the lumber. With the Perry device, an AC signal is converted to a DC signal prior to measurement of the signal strength as voltage. Perry differs from Applicant's invention in that Applicant directly measures the strength of the AC signal. Perry also does not employ a phase shift measurement to improve the moisture content estimate. In addition, the Perry device has no capability to estimate dielectric material density.
U.S. Pat. No. 4,580,233 to Parker et al. teaches an adjacent electrode AC moisture sensing device with two alternating frequencies that measures the imbalance in a capacitance bridge to estimate the moisture content of dielectric materials. Circuitry and methodology is incorporated to correct for potential wood temperature differences. As with the Lugwig et al., Burns, and Perry disclosures, the AC signal is converted to a DC signal prior to measurement of voltage to determine signal strength. This differs from Applicant's invention, which directly measures the strength of the AC signal. In addition, Parker et al. does not employ phase shift either to improve the moisture content estimate or to allow for estimation of dielectric material density.
U.S. Pat. No. 5,402,076 to Havener et al. recites a portable device, similar to Perry's device, that measures moisture content in a stack of lumber but with the AC radio frequency signal transmitted between adjacent electrodes. As with Perry, Applicant's invention differs because Applicant measures the phase shift and has the capability to estimate wood density.
U.S. Pat. No. 5,486,815 to Wagner discloses an in-line AC moisture meter employing opposed capacitance electrodes to sense moisture content in lumber moving between the electrodes. A single 4 MHz frequency is transmitted between electrodes and the received signal strength is measured to provide an estimate of the wood moisture content. The 4 MHz signal is applied to two pairs of electrodes with a 20-volt peak-to-peak amplitude signal applied to one pair and a 4.5 volt peak-to-peak amplitude signed to the other. The 4.5 volt signal is applied 180° out-of-phase with the higher 20-volt signal. Wagner teaches that analysis of the out-of-phase signal responses reduces the effects on the signal of electrical loading of the material. Wagner differs from Applicant's invention because Wagner does not improve the estimate of moisture content by adding phase shift information and Wagner has no described capability to estimate the dielectric material density. This device is also limited to detection of moisture content below 24 percent.
The teachings described above have employed measures of signal strength of both resistance and capacitance electrodes to estimate dielectric material moisture content. Both AC and DC devices have been developed. However, none of the described devices are reportedly accurate in measuring moisture content above the fiber saturation point of approximately 24 to 30 percent moisture content. In addition, none have employed measurement of signal phase shift to improve their estimate of moisture content. Furthermore, none report the capability of estimating the density of the dielectric material by combined analysis of amplitude and phase shift of a radio frequency signal.
U.S. Pat. No. 5,086,279 to Wochnowski discloses a means for estimating moisture content in a stream of materials by both reflecting and passing electrical energy through the stream in the form of infrared, microwave, or energy generated by a high-frequency oscillator circuit. For each of the electrical energy types, the energy is both reflected from and transmitted through the material stream. The transmitted energy from the high-frequency oscillator may be inferred to be in the same radio frequency range as Applicant's invention, although Wochnowski did not define the spectrum.
The Wochnowski moisture content estimate of the stream of materials depends on measures of signal strength and phase shift with each obtained by two methods. The two methods are to obtain a reflected signal detected by a sensor on the same side of the stream of materials and also a through signal such as is obtained by an opposed or adjacent electrode configuration. Therefore, the moisture content estimate provided by Wochnowski depends partially on the correction for the mass of the stream of materials by analysis of the “damping of oscillations” of electromagnetic waves through a first signal and a second (reflected) signal. Likewise, additional information for the moisture content analysis is obtained from the phase shift of both a through and reflected signal.
Applicant's invention differs from Wochnowski in that it requires no information on reflected energy but depends solely on its estimate of moisture content and density based on passage of the signal between the electrodes. In addition, Applicant compares phase shift and signal strength changes, caused by interaction of the radio frequency signal with the dielectric material, to predetermined values to provide the estimation of moisture content and density. Wochnowski describes no method for comparing predetermined values to correlate measured phase shift and signal strength decrease to expected values for the dielectric material at given moisture content's and densities. Applicant also provides an estimate of dielectric material density that the Wochnowski device does not provide.
In a 1993 writing, Torgovnikov discloses dielectric constants, measures of signal strength, and loss tangent values for radio frequencies from 20 to 1000 Hz. G. Torgovnikov, DIELECTRIC PROPERTIES OF WOOD AND WOOD-BASED MATERIALS 174–181 (1993). (The terms loss tangent and phase shift are both referenced herein. While these terms differ in their meaning, they are mutually direct functions with one easily derived from the other. For this reason, devices designed to provide information for one also indirectly provide the other value. In that sense these terms will be used interchangeably.) For all frequencies tested, Torgovnikov shows via plotted regressions that the rate of increase in the dielectric constant is higher for moisture content below the fiber saturation point. The plotted slopes of the regression lines also appear to have significant slope above the fiber saturation point. These plotted regression lines, however, represent the mean dielectric response for a range of wood specific gravity values.
Torgovnikov also teaches that the dielectric response is strongly influenced by the wood specific gravity. Therefore, dielectric constant information alone will not allow an accurate estimation of wood moisture content because of the confounding influence of wood density. With current methods this confounding influence can only be eliminated if a single wood density is scanned or if the density of specimens is known. Torgovnikov does not provides a method to improve moisture content estimate by including phase shift or loss tangent as a predictive variable.
Moisture Estimation Using Microwaves
Attempts have been made to measure the moisture content of materials using microwave energy. U.S. Pat. Nos. 4,727,311 and 5,767,685 to Walker teach ways to measure the moisture content of materials such as sand and coal. In these cases, two microwave frequencies are passed through a material in order to determine moisture content. The difference between the two signals assists in determining moisture content.
U.S. Pat. No. 4,674,325 to Kiyobe et al. calculates moisture content by passing material between non-contacting microwave horns. The basis weight is detected with an ionizing chamber.
U.S. Pat. No. 5,315,258 to Jakkula et al. discloses a radar system developed for measuring the moisture content of materials. There, the change in velocity of the microwaves within the material is correlated to differences in moisture content.
The Walker, Kiyobe et al., and Jakkula et al. teachings differ from Applicant's invention in that a microwave signal rather than signals in the radio frequency spectrum are utilized. Microwave devices require wave guides to transmit and receive the signals while radio frequency devices such as the Applicant's require only electrodes. These microwave devices described also do not have the capability to estimate dielectric material density.
The following disclosures describe microwave devices based on the attenuation of the microwave signal to estimate moisture content combined with information on phase shift of the microwave signal to provide wood density information:    R. King et al., Microwave Measurement of the Complex Dielectric Tensor of Anisotropic Slab Materials, in PROCEEDINGS OF A TECHNOLOGY AWARENESS SEMINAR (Nov. 15–16, 1987).    R. King et al., Measurement of Basis Weight and Moisture Content of Composite Boards Using Microwaves, in PROCEEDINGS OF THE 8TH SYMPOSIUM ON THE NONDESTRUCTIVE TESTING OF WOOD (Sep. 23–25, 1991).    P. Martin et al., Evaluation of Wood Characteristics: Internal Scanning of the Material by Microwaves, in 21 WOOD SCIENCE TECH. 367–371 (1987).    P. Martin et al., Industrial Microwave Sensors for Evaluation of Wood Quality, in FOURTH INT'L CONFERENCE ON SCANNING TECHNOLOGY IN THE WOOD INDUSTRY (1991).    J. Portala & J. Ciccotelli, Nondestructive Testing Techniques Applied to Wood Scanning, in 2 INDUSTRIAL METROLOGY 299–307 (1992).
King et al. (1987), King et al. (1983), Martin et al. (1987), Martin et al. (1991), and Portala et al. (1992) depend for their estimates of moisture content and density on the analysis of both attenuation and phase shift. These devices differ from applicant's device in that the microwaves are applied by horns, rather than by the electrodes utilized by Applicant's device. No electrode based radio frequency or microwave device has been disclosed that combines analysis of changes in signal amplitude and phase shift to estimate wood moisture content and density, with the exception of Wochnowski. As discussed, this device requires information on both reflected and through-material amplitude and phase shift signals to obtain estimated material moisture content.
Radio Frequency Moisture Gradient Estimation
An impedance detector disclosed by Tiitta et al. measures the moisture gradient in wood. M. Tiitta et al., Development of an Electrical Impedance Spectrometer for the Analysis of Wood Transverse Moisture Gradient, in PROCEEDINGS OF THE 12TH INT'L SYMPOSIUM ON NONDESTRUCTIVE TESTING OF WOOD (Sep. 13–15, 2000). Electrodes contained in a probe are placed on the wood surface. One electrode transmits an electrical signal at frequencies below 5 MHz, and the second receives the signal. A variable electric field is developed between the electrodes. Analysis of the behavior of impedance, or signal strength, for the various frequencies transmitted through the wood allows estimation of the moisture gradient within the wood. This device was developed to sense the moisture gradient in logs.
Writings by Sobue and Jazayeri et al. have demonstrated a method to sense the moisture gradient in wood by what Sobue termed Electrode Scanning Moisture Analysis (ESMA). N. Sobue, Measurement of Moisture Gradient in Wood by Electrode Scanning Moisture Analysis ESMA, in PROCEEDINGS OF THE 12TH INT'L SYMPOSIUM ON NONDESTRUCTIVE TESTING OF WOOD (Sep. 13–15, 2000); S. Jazayeri & K. Ahmet, Detection of Transverse Moisture Gradients in Timber by Measurements of Capacitance Using a Multiple-Electrode Arrangement, 50 FOREST PROD. J. 27–32 (2000). ESMA determines moisture content at various depths through wood thickness by manipulating the distance between adjacent electrodes on a single wood surface between 0.43 in. (11 mm) and 1.97 in (50 mm), shown in FIG. 1. Examination of the capacitance changes developed by manipulation of electrode distance allows computation of wood moisture gradient at various depths through wood thickness. Sobue's method allowed measurement of moisture content in wood up to 120 percent. Sobue and Jazayeri et al., however, demonstrated that this method would work for only a single wood density in which moisture content levels were manipulated.
The Tiitta et al., Sobue, and Jazayeri et al. devices are adjacent electrode impedance devices that are designed to estimate moisture gradient rather than average moisture content. The ability to estimate wood density as well as moisture gradient was not demonstrated by this device. By contrast, Applicant's invention is an opposed or adjacent plate capacitance device that senses mean moisture content and may also provide an estimate of wood density. Neither the Tiitta et al., Sobue, or Jazayeri et al. devices employ phase shift to improve their estimate of moisture content or to provide an estimate of wood density.
U.S. Pat. No. 5,585,732 to Steele et al. and two writings by Steele et al. have disclosed a method for detecting density differences in scanned lumber by a radio frequency method with opposed electrodes. P. Steele & J. Cooper, Estimating Strength Properties of Lumber with Radio Frequency Scanning, in PROC. OF THE 4TH INT'L CONFERENCE ON IMAGE PROCESSING AND SCANNING OF WOOD (Aug. 21–23, 2000); P. Steele et al., Differentiation of Knots, Distorted Grain, and Clear Wood by Radio-Frequency Scanning, 50 FOREST PROD. J. 58–62 (2000). To date, only detection of knots and voids has been described as being detected. The application of phase shift or loss tangent to assist in more accurately estimating dielectric material moisture content or estimating density has not been disclosed for this or any other radio frequency device.
The disclosures by Steele et al. employed dielectric properties and wood density in the estimation of wood strength by radio frequency capacitance employing a variation of the Steele et al. device. However, the Steele et al. method depended on prior knowledge of wood moisture content with statistical correction for the known moisture content differences. Validation of this method showed an R2 value of 0.67 between attenuated dielectric signal and lumber modulus of rupture. Only a single radio frequency signal attenuation measurement to provide specific gravity estimates was employed. Applicant's invention, by contrast, may employ single or multiple radio frequency signals to obtain dielectric constant. The Steele et al. method did not measure phase shift to improve the estimate of wood density.
Wood Strength Estimation Based on Density Detection
The amount of lumber graded by machine stress rating (MSR) has continued to increase since the development of the technology in the early 1960's. This growth has been driven by the significant premium in value for MSR versus visually graded lumber in certain lumber grades. MSR graded lumber is mechanically flexed to obtain a flatwise modulus of elasticity. In a 1968 writing, Muller teaches a method of estimating lumber grade based on the known relationship between modulus of elasticity and modulus of rupture combined with additional information from visual inspection of the lumber. P. Muller, Mechanical Stress-Grading of Structural Timber in Europe, North America and Australia, 2 WOOD SCI. & TECH. 43–72 (1968). In addition, in 1997 Biernacki et al. indicated a significant potential for increased lumber value from improved accuracy in lumber grading. R2 values based on relating modulus of elasticity to modulus of rupture are species dependent but are approximately 0.50. J. Biernacki et al., Economic Feasibility of Improved Strength and Stiffness Prediction of MEL and MSR Lumber, 47 FOREST PROD. J. 85–91 (1997).
U.S. Pat. No. 4,941,357 to Schajer discloses an alternative to MSR lumber grading that is a system that estimates lumber strength based on x-ray through-lumber-thickness scanning. By this method the lumber strength is estimated by assigning a clear wood strength value with deductions based on knot presence indicated by specific gravity scans. Lumber strength estimations based on x-ray scanning is reported to be higher than MSR estimates with R2 values ranging between 0.68 and 0.78 for southern yellow pine lumber.
Applicant's invention has potential as an MSR device capable of predicting clear wood density. In such use, Applicant's invention will require a knot detection system such as a digital camera, ultrasound, radio frequency, infrared, etc. MSR lumber grading requires information on knot size and location in addition to density of clear wood. Also required will be techniques and software to correct for knot influence on lumber strength.
Additional Information
Researchers have employed microwave horns and open-ended coaxial cables to detect the moisture content, density, and characteristics of biological tissue including wood. Some devices such as Walker (U.S. Pat. Nos. 4,727,311 and 5,767,685), Kiyobe et al. (U.S. Pat. No. 4,674,325) utilize horns to direct microwaves through material and determine the moisture content based on the microwave attenuation. Jakkula et al. (U.S. Pat. No. 4,674,325) disclose a radar system that passes microwaves through materials whereby moisture content is correlated to changes in microwave velocity. Applicant's invention, by contrast, utilizes microwave signals applied by electrodes rather than horns. Also, in addition to signal attenuation, phase shift information is utilized by Applicant to allow more accurate estimation of dielectric material moisture content and density.
Other researchers, listed below, have applied microwaves to wood with microwave horns. They have measured both the attenuation and phase shift of the microwave signal and have utilized the known relationship of these signals to estimate both wood density and moisture content. Applicant's invention is similar to these devices and methods described in that the combined information on signal attenuation and phase shift is utilized to estimate wood moisture content and density. However, Applicant's device employs electrodes rather than horns to apply and receive the microwaves.    R. King et al., Microwave Measurement of the Complex Dielectric Tensor of Anisotropic Slab Materials, in PROCEEDINGS OF A TECHNOLOGY AWARENESS SEMINAR (Nov. 15–16, 1987).    R. King et al., Measurement of Basis Weight and Moisture Content of Composite Boards Using Microwaves, in PROCEEDINGS OF THE 8TH SYMPOSIUM ON THE NONDESTRUCTIVE TESTING OF WOOD (Sep. 23–25, 1991).    P. Martin et al., Evaluation of Wood Characteristics: Internal Scanning of the Material by Microwaves, in 21 WOOD SCIENCE TECH. 367–371 (1987).    P. Martin et al., Industrial Microwave Sensors for Evaluation of Wood Quality, in FOURTH INT'L CONFERENCE ON SCANNING TECHNOLOGY IN THE WOOD INDUSTRY (1991).    J. Portala & J. Ciccotelli, Nondestructive Testing Techniques Applied to Wood Scanning, in 2 INDUSTRIAL METROLOGY 299–307 (1992).
Wochnowski (U.S. Pat. No. 5,086,279) has disclosed application of electromagnetic waves and simultaneous measurement of their transmission through, and reflectance from, materials. He utilized this information to estimate moisture content and density of the materials. Means of application and frequency was not specified. Applicant's invention does not utilize electromagnetic wave reflectance information to arrive at an estimate of dielectric material moisture content and density.
Numerous researchers, such as those listed below, have employed open-ended coaxial cables applying microwaves to measure dielectric properties of biological materials, including wood. Applicant's application of microwave signals is achieved with electrodes rather than with open-ended coaxial cables.    Atheny, T. W., M. A. Stuchly & S. S. Stuchly, 1982. Measurement of RF Permittivity of Biological Tissues with an Open-Ended Coaxial Line. IEEE. TRANS. MTT. 30:82–86.    Hagl, D. M., D. Popovic, S. C. Haguess, J. H. Brooke, & M. Okoniewski. 2003. Sensing Volume of Open-Ended Coaxial Probes for Dielectric Characterization of Breast Tissue at Microwave Frequencies. IEEE. TRANS. MTT. 51(4):1194–1206.    Kabir, M. F., W. M. Daud, K. B. Khalid & H. A. A. Sidek. 2001. Temperature Dependence of the Dielectric Properties of Rubber Wood. WOOD AND FIBER SCIENCE. 33(2):233–238.
Steele & Kumar (U.S. Pat. No. 5,585,732) describe a radio frequency knot and void detection device utilizing opposed electrodes to detect knots and voids.
Forrer & Funck have also utilized a variation of a device disclosed by Ichijo in 1953. The Ichijo device is a resonant guard electrode design for sensing dielectric properties of dielectric materials. Opposed electrodes are placed in direct contact with surface of the dielectric material. Forrer & Funck applied their variant of the device to detect wood types such as knots, clear wood, pitch streaks, pitch pockets, voids and blue stain. Moderate success in correlating these defect types with attenuation and phase shift were successful but considerable overlap was found among these measures depending on the wood types. Frequency applied was in the radio frequency range between 1.4 and 20 MHz and was, therefore, restricted to the radio frequency range. This work by Forrer & Funck and by Ichijo was reported in the following papers:    Forrer, J. & J. Funck. 1998. Dielectric Properties of Defects on Wood Surfaces. HOLZ ALS ROH-UND WERKSTOFF. 56(1):25–29.    Ichijo, B. 1953. On the New Method of Measuring Dielectric Constant and Loss Angles of Semiconductors. J. APPL. PHYS. 24(3):307–311.
Applicant's invention differs from the Forrer & Funck device in several important ways, including but not limited to the following. One important difference is that the Forrer & Funck device utilizes a resonance-based method and circuit to determine the dielectric constant and loss tangent of the wood types. The resonance device requires relatively complex circuitry comprising multiple resistors and capacitors. Also, the dielectric material scanned must be physically manipulated multiple times following calibration steps to determine the dielectric constant and loss tangent of the dielectric material. This method is obviously not suited to scanning wood to detect wood types for industrial purposes. Applicant's invention, by contrast, applies a discrete frequency and utilizes measurement devices to determine attenuation (related to dielectric constant) and phase shift (related to loss tangent). Thus, applicants method is adaptable to industrial scanning speeds.
Forrer & Funck also demonstrate no method for identifying wood types from the dielectric constant and loss tangent data. Neither regression equations nor threshold values based on predetermined values were utilized. Rather, dielectric constant values were merely plotted versus loss tangent values to indicate a graphical relationship and distribution of response to wood types. Thus, applicants method is adaptable to industrial scanning speeds.
Researchers, listed below, have reported on a method and device to detect knots and other anomalies in logs by analysis of depolarized reflected microwaves transmitted from a microwave horn. Knots and log pith were successfully visualized. This device differs from Applicant's method in that microwaves are applied by Applicant by electrodes rather than horns.    Kaestner, A. P. and L. B. Baath. 2000. Microwave Polarimetry Based Wood Scanning. PROC. OF THE 12th INT. SYMPOSIUM ON NONDESTRUCTIVE TESTING OF WOOD. University of Western Hungary, September 13–15. Sopron, Hungary. 474 p.
The patent titled “Determining the Dielectric Properties of Wood,” (DDPW), numbered PCT/US96/03604 with International Publication Number WO 96/28741 and International Publication Date Sep. 19, 1996, invented by Venter & Viljoen, provides no method for determining the density of the wood between the electrodes.
The patent titled “Dielectric Sensor Apparatus”, numbered U.S. Pat. No. 5,654,643, invented by Bechtel et al. senses dielectric materials utilizing cancellation of capacative coupling.